Authenticate Using Yahoo on Android

You can let your users authenticate with Firebase using OAuth providers like
Yahoo by integrating web-based generic OAuth Login into your app using the
Firebase SDK to carry out the end to end sign-in flow.

Before you begin

To sign in users using Yahoo accounts, you must first enable Yahoo as a sign-in
provider for your Firebase project:

Add the Client ID and Client Secret from that provider's developer console to the
provider configuration:

To register a Yahoo OAuth client, follow the
Yahoo instructions
on registering a web application with Yahoo.

When registering apps with these providers, be sure to register the
*.firebaseapp.com domain for your project as the redirect domain for your
app.

Click Save.

In your project-level build.gradle file, make sure to include Google's
Maven repository in both your buildscript and allprojects sections.

If you haven't yet specified your app's SHA-1 fingerprint, do so from the
Settings page
of the Firebase console. Refer to
Authenticating Your Client
for details on how to get your app's SHA-1 fingerprint.

Handle the sign-in flow with the Firebase SDK

If you are building an Android app, the easiest way to authenticate your users
with Firebase using their Yahoo accounts is to handle the entire sign-in flow
with the Firebase Android SDK.

To handle the sign-in flow with the Firebase Android SDK, follow these steps:

Construct an instance of an OAuthProvider using its Builder with the
provider ID yahoo.com.

For the parameters Yahoo supports, see the
Yahoo OAuth documentation.
Note that you can't pass Firebase-required parameters with
setCustomParameters(). These parameters are client_id,
redirect_uri, response_type, scope and state.

Optional: Specify additional OAuth 2.0 scopes beyond basic profile that
you want to request from the authentication provider. If your application
requires access to private user data from Yahoo APIs, you'll need to
request permissions to Yahoo APIs under API Permissions in the Yahoo
developer console. Requested OAuth scopes must be exact matches to the
preconfigured ones in the app's API permissions. For example if, read/write
access is requested to user contacts and preconfigured in the app's API
permissions, sdct-w has to be passed instead of the readonly OAuth scope
sdct-r. Otherwise,the flow will fail and an error would be shown to the
end user.

Authenticate with Firebase using the OAuth provider object. Note that unlike
other FirebaseAuth operations, this will take control of your UI by popping
up a Custom Chrome Tab.
As a result, do not reference your Activity in the OnSuccessListeners and
OnFailureListeners that you attach as they will immediately detach when the
operation starts the UI.

You should first check if you've already received a response. Signing in via
this method puts your Activity in the background, which means that it can be
reclaimed by the system during the sign in flow. In order to make sure that
you don't make the user try again if this happens, you should check if a
result is already present.

Where YAHOO_USER_UID is the Yahoo user's ID which can be parsed from
the firebaseAuth.getCurrentUser().getProviderData().get(0).getUid() field
or from authResult.getAdditionalUserInfo().getProfile().

While the above examples focus on sign-in flows, you also have the
ability to link a Yahoo provider to an existing user using
startActivityForLinkWithProvider. For example, you can link multiple
providers to the same user allowing them to sign in with either.

Advanced: Handle the sign-in flow manually

Unlike other OAuth providers supported by Firebase such as Google, Facebook,
and Twitter, where sign-in can directly be achieved with OAuth access token
based credentials, Firebase Auth does not support the same capability for
providers such as Yahoo due to the inability of the Firebase
Auth server to verify the audience of Yahoo OAuth access tokens.
This is a critical security requirement and could expose applications and
websites to replay attacks where a Yahoo OAuth access token obtained for
one project (attacker) can be used to sign in to another project (victim).
Instead, Firebase Auth offers the ability to handle the entire OAuth flow and
the authorization code exchange using the OAuth client ID and secret
configured in the Firebase Console. As the authorization code can only be used
in conjunction with a specific client ID/secret, an authorization code
obtained for one project cannot be used with another.

If these providers are required to be used in unsupported environments, a
third party OAuth library and
Firebase custom authentication
would need to be used. The former is needed to authenticate with the provider
and the latter to exchange the provider's credential for a custom token.

Next steps

After a user signs in for the first time, a new user account is created and
linked to the credentials—that is, the user name and password, phone
number, or auth provider information—the user signed in with. This new
account is stored as part of your Firebase project, and can be used to identify
a user across every app in your project, regardless of how the user signs in.